The gaseous nebula NGC 6888, also called the veil nebula, the crescent nebula,
Caldwell 27, Sharpless 105, is an asymmetric wind bubble found around the
evolved massive Wolf-Rayet WN66(h)-type star WR 136. Its particular shape is
made of two bipolar lobes and of an equatorial ring of denser material.
It has long been studied in great detail [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}.
Early observations of the surroundings
of NGC 6888 revealed a variation of the level ionization, traced by the
optical line ratios [NII]/H\(\alpha \)  in the many clumps along the surface of the shell,
witnessing an efficient mixing of materials preceding the formation of the Wolf-Rayet
shell [1]}, [4]}. The presence of dust and
enriched elements around Wolf-Rayet nebulae indicate that (i) they result from wind-wind
interaction of evolved material, (ii) one of the evolved winds is that of a cold,
dust-producing, supergiant phase of stellar evolution and (iii) the kind of detected
dust tells us that the wind from a main-sequence star
does not contribute to the shaping of the nebula.
NGC 6888 has been the site of the first observation of the aftermath of the CNO
cycle of nuclear reactions into the stellar core [5]},
and its progenitor star has been constrained to be a single, red-supergiant-evolving
star of \(25-40\, \rm M_{\odot }\)  [12]}, [5]}.
Hence, these Wolf-Rayet stars must probably have undergone a red supergiant
evolutionary phase, and, such scenario has been modelled using 1D and 2D
spherically-symmetric hydrodynamical simulations in [3]}.
However, if it is clear that WR136 has been a cold star prior to the Wolf-Rayet
phase [12]}, the origins of the progenitor's asymmetries are
still not understood.
